The present invention relates generally to electronic musical instruments and, more particularly, to an improved tone generator for a keyboard electronic musical instrument which is capable of producing ramp and pulse width waveform tone signals whose amplitudes and pulse widths are extremely stable over a relatively wide frequency range.
Tone generators are typically used in keyboard electronic musical instruments for producing tone signals having waveforms adapted for simulating the sounds of selected acoustical musical instruments. For example, a tone signal having a ramp waveform is normally used to simulate the sound produced by a piano while the sounds of a jazz guitar and a jazz flute are usually simulated by fixed pulse width and square-wave waveform tone signals respectively.
In the past, the tone generation function of keyboard electronic musical instruments has been implemented by a plurality of tone signal waveform generating circuits, each such circuit being dedicated to a single keyboard key and having components tailored to the corresponding specific frequency. More recently, time-shared tone generation systems have found widespread use in keyboard electronic musical instruments, such use being largely facilitated by the incorporation of microprocessor systems in the instruments. In a time-shared tone generation system a limited number of individual tone generators are provided, each tone generator comprising a multifrequency tone signal source operable in response to a plurality of keyboard keys. As a result, it is desirable that each of the tone generators be operable over a relatively wide frequency range without distorting the characteristics, e.g. amplitude, waveshape, etc., of the tone signal produced thereby. Prior art tone generators used in time-shared tone generation systems normally comprise relatively complex digital circuits which are not considered to be sufficiently stable over the extended frequency range involved for producing tone signals to a desired degree of accuracy.
It is therefore a basic object of the present invention to provide an improved tone generator for use in a keyboard electronic musical instrument embodying a time-shared tone generation system.
It is a more specific object of the invention to provide a tone generator of the foregoing type which is relatively noncomplex in construction and which is capable of producing tone signals whose waveform characteristics are extremely stable over a wide frequency range.